Speedy Earthquake Rides "Sonic Boom"

HARSHA BHAT
The latest evidence indicates that a few earthquakes can sustain powerful sonic boom–like effects that may enhance their destructive power. The earthshaking results come from analyzing the cracks from a strong 2001 earthquake in Tibet that caused a so-called super-shear rupture—meaning the earth split apart nearly twice as fast as normal—along a quarter of the temblor's 400-kilometer- (250-mile-) long fracture.

Until recently, seismologists were unsure that an earthquake could sustain such high speeds for more than a brief instant. Knowing how prolonged super-shear tremors work may allow researchers to better gauge the damage-dealing potential of faults such as the San Andreas, which passes through San Francisco along the California coast.

Earthquakes occur when tectonic plates rub against one another along junctures called fault lines, causing violent shudders that can tear the earth above a fault. Before the 1970s, researchers believed that a crack could only move as fast as a water wave–like ripple called a transverse wave, which clocks out at the so-called shear speed of three kilometers (1.9 miles) a second.

But they think something different can happen at faults in which two plates rub sideway against each other—instead of one slipping below the other. The rubbing produces waves that vibrate back and forth along the fault line faster than the shear speed.

These longitudinal, or soundlike waves can create a second, faster-moving gash in front of the slower transverse rupture. "It creates a shock wave which is just like a sonic boom, and those waves are what I'm concerned about," says seismologist Shamita Das of the University of Oxford.

Several studies published by Das and other research groups indicated that a magnitude 7.8 earthquake in Kunlunshan, Tibet, in 2001 zipped along at five to six kilometers (three to 3.7 miles) a second, or close to the speed of sound in rock. That's fast enough to travel from New York City to Boston in less than a minute.

Earth scientists Harsha Bhat and James Rice of Harvard University and their colleagues reported earlier this year that secondary cracks that had split off from the main fracture were consistent only with a super-shear shock wave propagating for 100 kilometers (62 miles).

In this week's Science, Das says the finding clinches the case for sustained super-shear ruptures. "What we realize now is an earthquake behaves a little bit like a car," she says. Given a straight path, it can accelerate to super-shear speed and maintain that speed until it has to turn through a bend in the fault.

A similar effect may have contributed to the destruction wrought by the 1906 San Francisco earthquake, she says.

Super-shear quakes may create more violent tremors farther from the fault line than shear quakes do. Bhat and a co-worker have submitted a follow-up paper suggesting that the Tibetan super-shear rupture initiated a one-two punch of dual shock waves radiating outward from the fault. "Our simulations predict large ground motions to distances of the order of a few tens of kilometers" around a fault as deep as the San Andreas, he says.

But Rice notes that such effects ought to be very brief and sensitive to the kind of rock they encounter, which may limit their force—"so we don't yet understand all their consequences for earthquake hazard."